C assimilation and plant productivity

1. C assimilation and plant productivity Stern, “Introductory plant biology,” 10th edn. Plant Physiol Biotech 3470 Lecture 16 Chapter 9 Thurs 23 March 2006

2. Plants provide the carbon needed by all organisms • We have seen that plants reduce C from the atmosphere • This reduced C becomes biomass which includes • Crops • Trees and other forest species • Grasses • Human activity alters the biosphere • This affects plant growth via increased atmospheric [CO2] • more available CO2 → more productivity (plant growth)? ≡ higher yield? • Yield concerns important to agriculture but also ecologically • Energy and nutrient flow within communities • How plants respond to stress • Upper limits to productivity: need to understand to feed the world • Productivity is usually expressed as a rate • e.g., 300 kg per ha per year

3. Productivity is dependent on C fixation Fig. 9.1 Plants having a higher growth rate will have a higher growth respiration rate • Primary productivity (PP) is the conversion of solar energy to organic matter by plants • Gross PP is total carbon assimilation by plants • But, some C is respired (30-60%!) • Therefore, we can define Net PP = Gross PP – respiration = biomass available to animals • Respiration rate is asignificant limitation to plant growth • We can distinguish two types of respiration that depend on its purpose Mature leaf (not actively growing) Very young, actively growing leaf • 1. Growth respiration → the carbon cost of growth; the amount of fixed C required in respiration to power growth via ATP synthesis (mitosis of rapidly dividing cells) • 2. Maintenance respiration → the amount of fixed C allocated to providing energy for processes not resulting in growth (normal metabolism)

4. High respiration rates limit productivity Total respiration In ryegrass, higher growth rate at lower respiration rate • Can in theory improve productivity by lowering respiration rate • In ryegrass, high growth rates found in ecotypes having low respiration rates • Therefore, more C available for growth • Can also manipulate components of respiration • knock out the alternative oxidase to increase yield (much less ATP synthesis!) • Caution! Many enzymes may be required in the field under stress conditions! (as in maintenance respiration) Fig. 9.2 • Complicated processes, many enzymes → to manipulate efficiently via genetic engineering requires a thorough knowledge of pathway biochemistry • Identify regulatory steps • Changing one pathway will likely impact others! (e.g. hexose-P metabolism)

5. Many environmental factors limit productivity Fig. 9.3 • These include • Nutrients • Water • Temperature • Let’s examine a few others in detail (light and CO2 levels) Light fluence rate • At low light, respiration > photosynthesis • At light compensation point, net CO2 exchange is zero Saturation rarely occurs in natural conditions • Here, respiration rate = photosynthetic rate • 10-40 μmol photons / m2 / s • C3 plant photosynthesis becomes light-saturated- usually due to other photosynthetic limitations (e.g., CO2 availability) • C4 plants do not light saturate • Continue photosynthesis even at low internal CO2 levels thanks to Kranz anatomy

6. Productivity also depends on CO2 availability • [CO2] in the atmosphere = 0.035% (v/v) = 350 ppm • Below CO2 saturation levels for C3 plants • Therefore CO2 often limiting • Except in C4 plants → saturate at ambient • Photosynthesis more dependent on intracellular [CO2] rather than ambient [CO2] • But, ambient ≈ intracellular if the stomata are open in C3 plants C3 plants increase max p’syn rate and CO2 sat’n level at high fluence atm Fig. 9.4 • Photosynthetic capacity is determined by the balance of CO2 fixation capacity by rubisco and e- transport capacity At low CO2 levels, photosynthetic rate is limited • Not enough CO2 to operate the PCR cycle quickly • The cycle backs up with lots of the other rubisco substrate ( ______ ) present

7. Photosynthetic capacity limits the C assimilation rate Fig. 9.5 Little CO2, lots of RuBP Little CO2, lots of RuBP At high CO2 levels, photosynthetic rate is limited by low RuBP levels • When there is lots of CO2, rubisco activity is saturated • Availability of RuBP here is limiting for photosynthesis • CO2 assimilation walks a line between these 2 limitations Lots of CO2, little RuBP • Regulating the size of the stomatakeeps photosynthesis in transition zone where neither RuBP levels or CO2 levels are limited • BUT… stomata are supposed to regulate water loss (transpiration rate)! –different theory! • CO2 enrichment used to increase productivity in greenhouses → ↑[CO2]causes upregulation of CO2 fixation (PCR cycle) enzymes • Too high [CO2] feedback limits photosynthesis (nutrient limitations) • Also: limitations of source (leaf) tissues to store photoassimilate prior to transport to sinks

8. Maximizing biomass production requires integration of complex processes • Intimate metabolic connections exist between these processes! NH3 reduction Amino acids for protein Powers growth Amino acids for enzymes • Recall that photosynthesis-derived energy powers C skeleton biosynthesis for anabolism via the hexose-P pool • The integration of metabolism makes manipulating it to increase productivity tricky!

9. How does primary production work on a global scale? • The Earth produces 172 billion tonnes of biomass per year • 68% from terrestrial ecosystems ~ 30% area • 32 % from marine ecosystems ~ 70% area • Therefore, terrestrial productivity ~ 5X marine • Due to differences in nutrient supply • Water: nutrients sink out of photosynthetic active zone • Land: plants retain more available nutrients in litter • Most productive: tropical forests • ~21% of total biomass in rainforests alone! • long growing season • Only ~5% produced via agriculture • limited suitable lands available

10. Improving productivity on marginal land is a key goal • “Green revolution”- new cereal crop strains BUT mostly improve productivity on land already suitable for agriculture • A much bigger challenge is to improve productivity on marginal land • Salt-stressed • Drought-stressed • Unsuitable temperatures • Being addressed through biotechnological approaches • Needed to feed growing population • Most biomass produced by forests→ but extensive deforestation constantly occurring! • Causes declines in forest biomass and world biomass just at the time we need to reduce [CO2]atm to reach our Kyoto agreements! • One solution - now intensive research efforts, including in Canada (BIOCAP)